According to incomplete statistics, the world's annual output of cement is more than 1.5 billion tons, suggesting that the amount of concrete is at least 4.5 billion m3. China's consumption of concrete is about 1.5 billion m3 every year. It is an effective, simple and economical way for adding water reducer to concrete to improve intensity and the other properties of concrete. In 2007, the consumption of concrete water reducer in China reached 2.85 million tons, including 2.67 million tons of water reducer. With the rapid development of the construction industry and engineering construction, the consumption of concrete is increasing, as well as the demand of the concrete water reducer.
As biomass resources, lignin is second only to cellulose in natural abundance, accounting for 20˜30 wt % of plants. The annual output of lignin in world is about 150 billion tons. Industrial lignin comes form chemical pulping. Pulping industries in China can produce 10 billion tons per year of industry lignin, mainly including lignosulfonate in acid pulping waste liquor and alkali lignin in alkaline pulping back liquor. Lignosulfonate is macromolecular compound, consisting of hydrophobic phenylpropane skeleton and hydrophilic sulfonate groups. As an amphiphilic molecule, lignosulfonate has the abilities of adsorption, dispersion and moistening to the solid particles. Lignosulfonate was first used as concrete water reducer in U.S. in 1935, and then used in China in 1970s. However, because of its low water-reducing rate, excessive air-entraining and poor dispersive property, lignosulfonate is generally used as a normal water reducer, or used as a component to compound with high-performance water reducers for improving the workability and air-entraining ability of concrete.
Alkaline pupling (lime method, caustic soda method, caustic soda—anthraquinone method, kraft method, etc.) is the main method that used in China's paper industry, accounting for over 90%. By cooking the raw material, such as wood, bamboo, straw, reed, bagasse, alpine rush and other plants, the cellulose is isolated for papermaking, but the lignin and some hemicellulose (degraded into sugars) which are left in the cooking liquid become the main ingredients of the pulping effluent (referred to as black liquor). For producing one ton of pulp, it will bring about 10 tons of 10 wt % black liquor containing 0.3˜0.5 tons of lignin at the same time. The main methods to deal with black liquor are: alkali recovery, acid precipitation, flocculation, concentration and drying, etc. Alkali recovery is the way to retrieve alkali and heat by concentrating dilute black liquor and then burning it. Acid precipitation retrieves the lignin by adjusting the pH of dilute black liquor to strong acidic condition with sulfuric acid. Flocculation retrieves the lignin by depositing the lignin from dilute black liquor with flocculants. Concentration and drying is concentrating dilute black liquor to more than 40 wt % and then spray-drying it to powder. The lignin retrieved from acid precipitation and flocculation is called alkaline lignin. Alkaline lignin has high purity and its molecular weight is relatively higher. The product obtained by concentration and drying is known as a black-liquor-power. The black-liquor-power has almost the same composition as dilute black liquor, cotaining 30˜50 wt % lignin, 20˜30 wt % alkali, 8˜15 wt % inorganic salts, 20˜30 wt % sugar, etc. Based on protection of environment and utilization of renewable resource, large-size enterprises generally use the method of alkali recovery, while small or medium-size enterprises mostly retrieve alkaline lignin by acid precipitation or gain the black-liquor-power from concentration and drying. Alkaline lignin is insoluble in waters under neutral condition, so it can't be used as water reducer. The use of black liquor is more difficult because of its complex components and multiple impurities. Thus, chemical modification is an important way to realize the utilization of resource from alkaline lignin and black liquor.
Sulfonatin modification is the most effective way to modify alkali lignin for improving its water solubility and dispersive ability. For example, Kamouna et al. prepared a water reducer by sulfonating alkali lignin from esparto grass pulp back liquor (A. Kamoun et al. “Evaluation of the Performance of Sulfonated Esparto Grass Lignin as a Plasticizer-Water Reducer for Cement,” Cement and Concrete Research, 2003, 33, 995-1003). The process of preparation was as follows: Needlegrass alkali lignin was first extracted from black liquor, and then separated and purified. Sodium sulfite and formaldehyde were added into the alkali lignin solution, and the solution pH was adjusted to 7˜9. The reaction was allowed to continue for 3˜6 h at 130˜160° C., and the final product was used as a concrete water reducer wherein the water-reducing rate was 7%-12%. Nakano et al prepared a water reducer from rice-straw pulp back liquor by sulfomethylation, purification and compounding (J. Nakano et al. “Chemical Pulping of Straws: Preparation of Lignin Dispersant from Black Liquor of Rice-Straw Soda Cooking,” Jpn. Tappi J., 1995, 49, 1079-1085). Roy et al. gained sulfonated alkali lignin from bagasse back liquor, rice straw back liquor, and wheat straw back liquor by two steps of sulfonation at 140˜150° C., and the resulting sulfonated alkali lignin has a good dispersive property (T. K. Roy et al. “Utilization of Lignin from Agricultural Residues in the Manufacture of Dispersants,” IPPTA Seminar, 1989, 1-12). Matsushita and Yasuda obtained sulfonated alkali lignin by first activating the alkaline lignin with phenol, and then modifying it by sulfomethylation, hydroxymethylation and aryl sulfonation (Y. Matsushita, S. Yasuda, “Reactivity of a Condensed-type Lignin Model Compound in the Mannich Reaction and Preparation of Cationic Surfactant from Sulfuric Acid Lignin,” J. Wood Sci. 2003, 49, 166-171). Chang et al. prepared a water reducer by sulfonation of the alkaline lignin from wheat straw pulp black liquor wherein the water-reducing rate was more than 8% (D. Y. Chang et al. “Preparation of Concrete Water reducer by Wheat Straw Pulp Black Liquor,” Concrete, 1993, 5, 32-36). Fan et al. also obtained a sulfonated water reducer possessing a water-reducing rate of 10% (Y. B. Fan et al. “Development of a Concrete Water-Reducing Admixture Using Lignin from Alkaline Wheat Straw Pulping Black Liquor,” Chinese Journal of Enviromental Science, 1995, 4, 46-48).
U.S. Pat. No. 4,239,550 discloses flowing agents for mortar and concrete comprised of mixture of sulfonated lignin and sulfomethylated or ring-sulfonated aromatic compounds. U.S. Pat. No. 4,367,094 discloses slump reduction-preventing agents which mainly contain a sulfonated lignin having a carboxyl group content of at least 0.2 mols and a sulfonic group content of at least 0.1 mols per phenyl propane unit. CN 02100805.1 discloses concrete admixtures of sulfonated lignin prepared by reaction of straw pulping black liquor with formaldehyde and sulfite. CN 00131005.4 discloses a cement dispersant prepared by introducing carboxyl groups through electrooxidation of lignin, and then introducing sulfonic groups through sulfomethylation of lignin at 100˜175° C. CN 200510032657.7 discloses a method of preparing a high-performance water reducer by oxidation, sulfonation, condensation of alkaline lignin. CN 200410044834.9 discloses a method of preparing a sulfonated lignin water reducer though concentration, catalytic sulfonation, and compound of black liquor wherein the performance of the water reducer is in line with GB8076-1997 first-class product standards.
There are other reports about preparing water reducers by first sulfonating alkali lignin and then grafting with other high-performance water reducer. CN 200710051834.5 discloses a preparation method of a high-performance water reducer comprising a graft polymerization of herbal lignin sulfonate and naphthalene sulfonic acid wherein the weight ratio of lignin sulfonate to reactant naphthalene is 20%-240%. CN 200710130819 discloses a high-performance water reducer wherein 10 wt % dilute black liquor is modified by pre-homogenizing and sulfonating, and then treated by grafting with carbonyl aliphatic compounds. The water reducing rate of this water reducer is up to 22˜26%.
Although sulfonated alkaline lignin (containing the sulfonation product of black liquor) has advantages of abundant natural resources and low cost, it can only be used as normal water reducer because of its poor dispersive property. It was found that sodium lignosulfonate recoveried from acid pulping waste liquor has a weight-average molecular weight of 5000, and the degree of sulfonation is less than 1.5 mmol/g; the degree of sulfonation of sulfonated alkaline lignin is up to 2.0˜2.5 mmol/g (sulfonation degree is up to 3.0 mmol/g under microwave irradiation), but its weight-average molecular weight is less than 3500. Compared with sodium lignosulfonate, sulfonated alkaline lignin has higher sulfonation degree, but less molecular weight. Thus, the application of the sulfonated alkaline lignin is limited because its dispersive property is worse than sodium lignosulfonate.
The reason why sulfonated alkali lignin cannot become a high-performance water reducer is that the degree of the sulfonation and the molecular weight cannot be increased simultaneously. The structural characteristics of alkaline lignin lie in its high degree of condensation, and the ortho-position and para-position of phenolic hydroxyl in the stucure of guaiacyl and syringyl are occupied by methoxyl and side chains. Since the content of methoxyl is high, the active points for sulfonation and condensation reaction are relatively low. Therefore it is very difficult for sulfonated alkali lignin to elevate the degree of sulfonation and the molecular weight simultaneously.